Fix "grep: Version.h: No such file or directory" error when including the core LUFA library makefile in a legacy makefile.

LUFA/makefile

@@ -9,7 +9,7 @@
 #  Makefile for the LUFA library itself.
 # ---------------------------------------
 
-LUFA_VERSION_NUM        := $(shell grep LUFA_VERSION_STRING Version.h | cut -d'"' -f2)
+LUFA_VERSION_NUM         = $(shell grep $(LUFA_PATH)/LUFA_VERSION_STRING Version.h | cut -d'"' -f2)
 EXCLUDE_FROM_EXPORT     := Documentation DoxygenPages CodeTemplates Build StudioIntegration *.conf *.tar *.o *.d *.lss *.lst *.hex *.elf *.hex *.eep *.map *.bin
 
 # Default target - no default action when attempting to build the core directly